1. Field of the Invention
The present invention generally relates to aspheric surfaces and, more particularly, to a method and a system for characterizing the aspheric surfaces of optical elements.
2. Background Art
Null testing is a well known technique used to measure with precision flaws or defects of optical elements. In null testing, two wavefronts divided from one propagating light wave of constant phase are directed along separate optical paths, i.e., the test path and the reference path. The wavefront of the test path, i.e., the test wavefront, is directed toward an-optical surface to be characterized. The test wavefront will then be returned from the test path to an interferometer, wherein it will be reunited with the wavefront of the reference path, i.e., the reference wavefront, in an interference analysis.
The test path and the reference path are arranged such that, if the aspheric surface is without detectable irregularities, the test and reference wavefronts will be the same upon reaching the interferometer, i.e., the differences between the test and the reference wavefronts should be “null.” In practice, an aspheric surface may have some detectable irregularities that will cause deviation of the test wavefront in the test path. Any deviation in the wavefront will produce fringe distortions in an interference pattern with the wavefront of the reference path. As known in the art, the fringe distortions in the interferometer are readily interpretable, e.g., using a wavefront analyzer with appropriate software, so as to identify with great precision the irregularities in the aspheric surface of the optical element, and thus characterize the latter.
There are a few null testing systems that differ in test path configurations. Referring to FIG. 1 of the prior art, a portion of a test path of a known optical null testing system is generally shown at 10. The test path portion 10 has a reflective mirror 11 with a concentric aperture 12 and a reflective surface 17. The surface to be tested is generally shown at 13 and consists of an aspheric surface of a concave mirror. A test wavefront 14 converges out of the aperture 12 onto the test surface 13. The direction of the wavefront 14 going to the reflective surface 17 is shown by arrowheads 15. The aperture 12 is positioned such that the wavefront 14 is collimated upon being reflected by the test surface 13. The reflective surface 17 of the reflective mirror 11 is shaped as a function of the test surface 13. More precisely, every point on the reflective surface 17 is positioned tangentially with respect to the incoming wavefront 14, as anticipated from the theoretical shape of the test surface 13. If the test surface 13 does not have detectable flaws, i.e., the actual shape of the test surface 13 is the same as the theoretical shape, the wavefront 14 should return taking the same path, although in reversal, as shown by arrowheads 16. However, if there are detectable surface irregularities on the test surface 13, portions of the wavefront 14 will be deflected from the expected path and will thus not return in the reverted direction. Therefore, the wavefront 14 exiting through the aperture 12 will exclude the portions of the wavefront 14 that were deflected by the surface irregularities of the test surface 13. The wavefront 14 exiting from the aperture 12 with the reference wavefront (not shown) will be united in an interferometer. Interference fringes will appear for the wavefront portions that have not returned from the test path portion 10. The interpretation of the interference fringes will enable the precise identification of the irregularities on the test surface 13.
One disadvantage of null testing in its various systems is that a reflective mirror being precisely shaped (i.e., shaped to the level of precision that is desired) as a function of the tested aspheric surface to be tested must be provided. This requires that a reflective mirror or equivalent surface (e.g., computer-generated hologram) be provided for each different shape of aspheric surface. Optical elements of specific dimensions are costly to manufacture, and thus the characterization of aspheric surfaces is a costly procedure. Accordingly, users of aspheric surfaces often rely on the manufacturers of the aspheric surfaces for the characterization thereof.